Nonaqueous electrolyte secondary battery

ABSTRACT

A nonaqueous electrolyte secondary battery which comprises a positive electrode including particles of lithium-containing layered nickel oxide represented by a general formula Li a Ni x Co y Al z M b O 2 , wherein 0.3≦a≦1.05, 0.7≦x≦0.87, 0.1≦y≦0.27, 0.03≦z≦0.1, 0≦b≦0.1; M is at least one selected from metallic elements except Ni, Co and Al. In the binding energy of the oxygen 1s spectrum when measuring the particles by XPS, if the peak area appearing at 529 eV is set to D; the peak area appearing at 531 eV is set to E; oxygen concentration ratio is set to D/(D+E); and the oxygen concentration ratios at depths of L1 nm and L2 nm from the particle surface are respectively set to α L1  and α L2 , the combination of L1 and L2 in which (α L2 −α L1 )/α L2 ≦0.1, L1≦100, L2≧500 is present.

TECHNICAL FIELD

The present invention relates to nonaqueous electrolyte secondarybatteries provided with a positive electrode which includeslithium-containing layered nickel oxide particles.

BACKGROUND ART

As electronic devices are rapidly reduced in size and weight, the demandis growing for batteries as a power source of electronic devices,secondary batteries which are small and lightweight, have high energydensity and further, are repeatedly chargeable and dischargeable shouldbe developed. Also, owing to environmental issues such as air pollutionand the increase in carbon dioxide, an early practical application in anelectric vehicle is anticipated. Therefore, there is a demand for thedevelopment of superior secondary batteries which have characteristicssuch as high efficiency, high power, high energy density and lightweight.

As a secondary battery which satisfies these demands, the secondarybattery which employs nonaqueous electrolyte has been put to practicaluse. The battery has several times higher energy density than aconventional battery which uses aqueous solution electrolyte. Forexample, a long-life, 4-volt class nonaqueous electrolyte secondarybattery has been put to practical use. It employs lithium-containinglayered cobalt oxide (hereinafter, Co-based compound),lithium-containing layered nickel oxide (hereinafter, Ni-based compound)or spinel-type lithium manganese composite oxide (hereinafter, Mn-basedcompound) for its positive electrode, and employs carbon material or thelike which can absorb and release lithium for the negative electrode.

Among them, Ni-based compound, characterized in that its amount of thelithium which can be absorbed and desorbed within a potential rangepractically used in the nonaqueous electrolyte secondary battery(3.0-4.3V vs. Li/Li⁺) is equal to or larger than the cases of Co-basedcompound and Mn-based compound, also thanks to its availability, haslargely been developed, aiming at a high-capacity and low-cost battery.

As described in Japanese Patent Publication No. H10-092429A, Ni-basedcompound is inherently more difficult to form so as to produce in alarge amount thereof having a homogeneous crystal structure, comparedwith Co-based compound, which has been widely adopted. Due to subsequentimprovements, however, examples are recently reported in which Ni-basedcompound incorporated into an actual battery delivers excellentperformance (see Journal of Power Sources 119-121 (2003) 859-864,865-869).

DISCLOSURE OF THE INVENTION

However, even today, when improvements have advanced, since a batteryusing Ni-based compound for positive active material is subject tovariation in performance, leaving concern about quality and reliabilitycompared with a conventional battery, and making its actualcommercialization is difficult.

Investigation of Ni-based compound from various angles in order todetermine the cause of such a problem shows that battery performancevaries greatly depending on slight differences in surface properties,which cannot be defined as general quality control items such ascomposition molar ratio, specific surface area, pH, bulk density, tapdensity, particle size distribution, impurity amount, particle shape andcrystal structure.

Accordingly, an object of the present invention is to provide anonaqueous electrolyte secondary battery having a large dischargecapacity and excellent charge-discharge cycle characteristics byprescribing the surface properties of Ni-based compound particles whichhave a large impact on battery performance and by using a compound whosesurface state is within the stipulated range for positive activematerial of the nonaqueous electrolyte secondary battery.

In order to achieve the above objective, the nonaqueous electrolytesecondary battery according to the present invention is a nonaqueouselectrolyte secondary battery provided with a positive electrodeincluding particles of lithium-containing layered nickel oxiderepresented by the general formula Li_(a)Ni_(x)Co_(y)Al_(z)M_(b)O₂, inwhich 0.3≦a≦1.05, 0.7≦x≦0.87, 0.1≦y≦0.27, 0.03≦z≦0.1, 0≦b≦0.1; M is atleast one selected from metallic elements except Ni, Co and Al; in thebinding energy of the oxygen 1s spectrum when measuring the particles byXPS, the peak area appearing at 529 eV is set to D and the peak areaappearing at 531 eV is set to E; oxygen concentration ratio is set toD/(D+E); and the oxygen concentration ratios at depths of L1 nm and L2nm from the particle surface are respectively set to α_(L1) and α_(L2),in which case, the combination of L1 and L2 is present in which(α_(L2)−α_(L1))/α_(L2)≦0.1, L1≦100, L2≧500.

According to the present invention, the composition of Ni-based compoundparticles is stipulated as a whole so that the improvement effectproposed by the conventional inventions is obtained. Also, a compound inwhich the chemical bond state of oxygen close to the particle surface iswithin a prescribed range is used for positive active material of thenonaqueous electrolyte secondary battery so that the interfaceresistance between the electrolyte and the Ni-based compound particlebecomes small and charge-discharge repetition minimally increases theresistance, thereby bringing good battery performance for a long period.The present invention, which can consistently provide a nonaqueouselectrolyte secondary battery superior in discharge capacity and lifeproperty to a conventional battery using Co-based compound, is of highindustrial value.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a relation between a depth L (nm) from a particle surfaceand oxygen 1s spectrum in XPS spectrum;

FIG. 2 shows the oxygen 1s spectrum on the particle surface (L=1 nm) inXPS;

FIG. 3 shows the oxygen 1s spectrum at a depth L of 500 nm from thesurface in XPS;

FIG. 4 shows an example of the oxygen 1s spectrum of lithium-containinglayered nickel oxide particles used for positive active material of thepresent invention by XPS;

FIG. 5 shows a relation between L and (α₀−α_(L))/α₀ in thelithium-containing layered nickel oxide particles used for positiveactive material in the present invention;

FIG. 6 is a perspective view showing an appearance of a long cylindricalnonaqueous electrolyte secondary battery; and

FIG. 7 is a perspective view showing a configuration of an electrodegroup accommodated in the long cylindrical nonaqueous electrolytesecondary battery.

PREFERRED MODE FOR CARRYING OUT THE INVENTION

A nonaqueous electrolyte secondary battery which uses particles oflithium-containing layered nickel oxide represented by the generalformula Li_(a)Ni_(x)Co_(y)Al_(z)M_(b)O₂ (0.3≦a≦1.05, 0.7≦x≦0.87,0.1≦y≦0.27, 0.03≦z≦0.1, 0≦b≦0.1; and M is at least one selected frommetallic elements except Ni, Co and Al) for positive active material issubject to variation in discharge capacity, power performance,preserving performance or the like in production lots. The presentinventors investigated the cause in detail and clarified as follows:Even when the average composition of a compound as a whole is within therange stipulated by the above general formula, the chemical bond stateof elements close to the compound surface is often disturbed. The reasonremains unclear. Particularly, the variation in oxygen bond state isremarkable and corresponds to the variation in battery performance.

Consequently, the present inventors analyzed the surfaces of all theNi-based compounds formed as a trial under the same conditions as wereused in the past and selected a compound in which the oxygenconcentration profile on the surface is within a prescribed range so asto obtain a large-capacity and long-life nonaqueous electrolytesecondary battery. Hereinafter, a description is given for a concretemode of the nonaqueous electrolyte secondary battery according to thepresent invention.

The composition of lithium-containing layered nickel oxide particlesused for positive active material in the present invention as a whole isrepresented by the general formula Li_(a)Ni_(x)Co_(y)Al_(z)M_(b)O₂(0.3≦a≦1.05, 0.7≦x≦0.87, 0.1≦y≦0.27, 0.03≦z≦0.1, 0≦b≦0.1; and M is atleast one selected from metallic elements except Ni, Co and Al). In thelithium-containing layered nickel oxide particles, cobalt substitutesfor a part of nickel, thereby suppressing change in crystal structurecaused by charge-discharge process. Furthermore, trivalent stablealuminum is doped thereby giving more stability to the crystalstructure. In this case, M is at least one selected from metallicelements except Ni, Co and Al, and a combination of a plurality ofmetallic elements may be employed.

In addition, when charging up to an area of a<0.3, there is asubstantial change in c-axial length, thereby acceleratingdisintegration of the crystal structure and increasing polar plateresistance. Therefore, charging is detrimental up to such an area.Discharging, also for the same reason, within the range of a≦1.05 ispreferable.

Furthermore, when x is less than 0.7, the discharge capacity decreasesto be equivalent to a conventional nonaqueous electrolyte battery usinga Co-based compound for positive active material. When exceeding 0.87,heat stability decreases extremely. Therefore, x is preferably withinthe range between 0.7-0.87. Also, when y is less than 0.1, the crystalstructure becomes unstable. On the other hand, when y exceeds 0.27, thecrystal structure reaches the limit of stabilization, only to cause adecrease in discharge capacity. Therefore, y is preferably with in therange between 0.1-0.27. When z is less than 0.03, the crystal structurebecomes unstable and heat stability decreases in charging. However, whenz exceeds 0.1, discharge capacity decreases extremely. Therefore, Z ispreferably within the range between 0.03-0.1. In the present invention,0.98≦x+y+z+b≦1.01 is preferably valid. M is preferably a transitionmetal element except Ni and Co. For a transition metal element in thiscase, a combination of a plurality of transition metals may be used.

The reason for the variation in the concentration profile of oxygenclose to the surface of lithium-containing layered nickel oxideparticles remains unclear, but the following reasons are considered: dueto insufficient mixture or large particle size of the raw material, aresidue of unreacted raw material (lithium hydroxide or the like) isadhered to the compound surface after calcinations; a compound absorbsmoisture or carbon dioxide so as to form a lithium compound (lithiumcarbonate or the like) on the surface and furthermore absorbs moisturein a solvent when manufacturing a battery so as to form a lithiumcompound; the calcination temperature is out of the correct range(depending on compositions, commonly 650-750° C.); and calcination timeis too short. These reasons are considered to be due to the fact that itis inherently difficult to form a Ni-based compound and that the crystalstructure itself is unstable.

Even when supervising the manufacturing method and storage method of acompound and controlling the quality of the finished product asdescribed above with the full understanding that a Ni-based compound isdifficult to form and has a relatively unstable crystal structure, thesurface properties slightly vary in production lots and the variation inoxygen chemical bond state is often found from the surface toward theinside of the compound. Use of a compound whose surface propertiesdepart from a stipulated value for positive active material results inmanufacturing a battery unexpectedly poor in discharge capacity and lifeperformance.

In order to avoid such situation, the nonaqueous electrolyte secondarybattery of the present invention comprises a positive electrodeincluding lithium-containing layered nickel oxide particles representedby the general formula Li_(a)Ni_(x)Co_(y)Al₂M_(b)O₂ (0.3≦a≦1.05,0.7≦x≦0.87, 0.1≦y≦0.27, 0.03≦z≦0.1, 023 b≦0.1; M is at least oneselected from metallic elements except Ni, Co and Al). In the bindingenergy of the oxygen 1s spectrum when measuring the lithium-containinglayered nickel oxide particles by XPS, if the peak area appearing at 529eV is set to D; the peak area appearing at 531 eV is set to E; oxygenconcentration ratio is set to D/(D+E); and the oxygen concentrationratios at depths of L1 nm and L2 nm from the particle surface arerespectively set to α_(L1) and α_(L2), the combination of L1 and L2 inwhich (α_(L2)−α_(L1))/α_(L2)≦0.1, L1≦100, L2≧500 is present. In order touse such lithium-containing layered nickel oxide particles, sufficientconsideration has to be given to its raw materials, calcination method,handling method and the like, as a matter of course. On top of that,quality controls including examinations on the compound surface by Augerelectron spectroscopy, X-ray photoelectron spectroscopy (XPS), time offlight secondary ion mass spectrometry or the like are required. In thelithium-containing layered nickel oxide. particles according to thepresent invention, the combination of L1 and L2 in which the value of(α_(L2)−α_(L1))/α_(L2) is less than zero may be present, andparticularly, the combination of L1 and L2 equal to −0.1 or above ispreferably present. It is particularly preferable that the combinationof L1 and L2 in which the value of (α_(L2)−α_(L1))/α_(L2) is zero orabove and 0.1 or below is present.

FIGS. 1 to 3 show an example of the oxygen 1s spectrum of conventionallithium-containing layered nickel oxide particles by XPS. FIG. 1 shows arelation between the depth L (nm) from a particle surface and the oxygen1s spectrum when particles are argon-etched for a prescribed period oftime and the XPS spectrum is then measured repeatedly. FIG. 2 shows theoxygen 1s spectrum on the particle surface (L=1 nm); and FIG. 3 showsthe oxygen 1s spectrum at a depth L of 500 nm from the surface. Itshould be noted that there is no variation in the oxygen 1s spectrumfrom L=500 nm up to the core of the particle although omitted in FIG. 1.Since gases or impurities are slightly absorbed onto the surfaces of thelithium-containing layered nickel oxide particles, the spectrum afteretching down to a depth of 1 nm from the outermost surface is defined asa true surface spectrum here.

In FIGS. 1 to 3, the peak d is a peak appearing at 529 eV, showing thebinding energy of the oxygen 1s spectrum and the peak e is a peakappearing at 531 eV, showing the binding energy of the oxygen 1sspectrum. The peaks of the oxygen 1s spectrum are then separated. Thearea of the peak d is set to D; the area of the peak e is set to E; andthe oxygen concentration ratio α is defined as α=D/(D+E).

As described in the reference (K. Kanamura et al. J. Electroanal.Chemistry 419 (1996) 77-84) , the peak D represents oxygen contained inthe crystal of LiCoO₂ having a layer structure, that is O² ion in thecrystal; and the peak e represents the oxygen adsorbed onto the surfaceof the electrode. Therefore, in the XPS spectrum of the conventionallithium-containing layered nickel oxide particles shown in FIGS. 1-3,the peak d and the peak e represent the same things, and the area D ofthe peak d and the area E of the peak e clearly represent their oxygenconcentrations respectively.

The oxygen 1s spectrum on the particle surface (L=1 nm) shown in FIG. 2is D<E, showing that the concentration of the adsorbed oxygen is higherthan the concentration of oxygen contained in the crystal. The oxygen 1sspectrum at a depth of L=500 nm from the surface shown in FIG. 3 is D>E,showing that the concentration of oxygen contained in the crystal ishigher than the concentration of the adsorbed oxygen.

In the lithium-containing layered nickel oxide particles represented bythe general formula Li_(a)Ni_(x)Co_(y)Al_(z)M_(b)O₂ (0.3≦a≦1.05,0.7≦x≦0.87, 0.1≦y≦0.27, 0.03≦z≦0.1, 0≦b≦0.1; M is at least one selectedfrom metallic elements except Ni, Co and Al), used for the positiveactive material of the present invention, in the binding energy of theoxygen 1s spectrum when measuring the particles by XPS, if the peak areaappearing at 529 eV is set to D; the peak area appearing at 531 eV isset to E; oxygen concentration ratio is set to D/(D+E); and the oxygenconcentration ratio at depths of L1 nm and L2 nm from the particlesurface are respectively set to α_(L1) and α_(L2), a combination of L1and L2 in which (α_(L2)−α_(L1))/α_(L2)≦0.1, L1≦100, L2≧500 is present.

FIG. 4 shows an example of the oxygen 1s spectrum of lithium-containinglayered nickel oxide particles used for positive active material of thepresent invention by XPS. In FIG. 4, both the oxygen 1s spectrum on thesurface (L=1 nm) and the oxygen 1s spectrum at a depth of L=500 nm fromthe surface are D>E, which represents that the concentration of theoxygen contained in the crystal is higher than the concentration ofoxygen adsorbed anywhere in the particle regardless of the depth fromthe surface.

FIG. 5 shows the relation between L and (α₀−α_(L)/α) ₀ in thelithium-containing layered nickel oxide particles used for positiveactive material according to the present invention. Here, α₀ means α_(L)at an arbitrary depth of 500 nm or deeper from the particle surface. Inthe example of the figure, α_(L) is equal to α₀ at any depth deeper than500 nm from the surface. In this case, the lithium-containing layerednickel oxide particles used for positive active material according tothe present invention has L in which (α₀−α_(L))/α₀≦0.1 at a depth of 100nm or below from the surface is present, to which the curves X and Yshown in FIG. 5 correspond. The lithium-containing layered nickel oxideparticles shown as the curve Z in FIG. 5, having no L in which(α₀−α_(L))/α₀≦0.1 when 100≦L, is not included in the present invention.Although the curves X, Y and Z have the same value for (α₀−α_(L))/α₀when L=0, the relation between the present invention and theconventional example is not limited to such a case. In thelithium-containing layered nickel oxide particles according to thepresent invention, generally, the value of (α₀−α_(L))/α₀ when L=0 issmaller than the conventional example.

The relation between L of the lithium-containing layered nickel oxideparticles used for positive active material according to the presentinvention and (α₀−α_(L))/α₀ can be changed by the temperature and timefor precursor calcination after forming the precursor of thelithium-containing layered nickel oxide particles and furthermore, bythe atmosphere and time for storage.

The average particle size D₅₀ of the lithium-containing layered nickeloxide particles used for positive active material according to thepresent invention is preferably 4-20 μm and particularly preferably 9-10μm. The size range of the particle is preferably 2-30 μm. The BETspecific surface area of the particle is preferably 0.1-1 m²/g andparticularly preferably 0.3-0.4 m²/g. It should be noted that theaverage particle size and the particle size range were measured usingthe laser diffraction/scattering method and that the BET specificsurface area was measured using the BET method.

The nonaqueous electrolyte secondary battery according to the presentinvention, as shown in FIG. 6 and FIG. 7, is configured by accommodatingan electrode group formed by winding positive and negative electrodesusing the compound described above for positive active material into acircle or ellipse through a separator, in a battery case; and byimpregnating the electrode group with nonaqueous electrolyte.

FIG. 6 is a perspective view showing an appearance of a long cylindricalnonaqueous electrolyte secondary battery; and FIG. 7 is a perspectiveview showing a configuration of the electrode group accommodated in thelong cylindrical nonaqueous electrolyte secondary battery. In FIG. 6 andFIG. 7, reference numeral 1 denotes a nonaqueous electrolyte secondarybattery; 2 a power generating element; 2a a positive electrode; 2b anegative electrode; 2c a separator; 3 a battery case; 3a a case of thebattery case; 3b a lid of the battery case; 4 a positive electrodeterminal; 5 a negative electrode terminal; 6 a safety valve; and 7 anelectrolytic solution inlet.

For the negative electrode, separator, electrolyte and the like to beused for the nonaqueous electrolyte secondary battery, conventional oneswhich have been used normally can be used, since there is no particulardifference. Specifically, for the material of the negative electrode ofthe nonaqueous electrolyte secondary battery according to the presentinvention, various carbon materials capable of absorbing and desorbinglithium ions, metallic lithium, lithium alloy or the like can be used.Transition metal oxides or nitrides may also be used.

Also, for the separator used for the nonaqueous electrolyte secondarybattery according to the present invention, a microporous film composedof a polyolefin resin such as polyethylene is used. The separator may becomposed by laminating a plurality of microporous films different inmaterial, average molecular weight and porosity, or by applying a properamount of an additive such as various plasticizers, anti-oxidants andfire retardant to the microporous films.

The organic solvent of the electrolytic solution used for the nonaqueouselectrolyte secondary battery according to the present invention is notlimited to a specific one. For example, ethers, ketones, lactones,nitrites, amines, amides, sulfur compound, halogenated hydrocarbons,esters, carbonates, nitro compound, phosphate-based compounds andsulfolane-based hydrocarbons can be used. Among them, ethers, ketones,esters, lactones, halogenated hydrocarbons, carbonates andsulfolane-based compounds are preferable. The examples of the preferableorganic solvent include tetrahydrofuran, 2-methyltetrahydrofuran,1,4-dioxane, anisole, monoglyme, 4-methyl-2-pentanone; ethyl acetate,methyl acetate, methyl propionate, ethyl propionate, 1,2-dichloroethane,γ-butyrolactone, dimethoxyethane, methyl formate, dimethyl carbonate,methyl ethyl carbonate, diethyl carbonate, propylene carbonate, ethylenecarbonate, vinylene carbonate, dimethyl formamide, dimethyl sulfoxide,dimethylthioformamide, sulfolane, 3-methyl-sulfolane, trimethylphosphate, triethyl phosphate and a mixture of these solvents, but notlimited to the above examples. Cyclic carbonates and cyclic esters arepreferable. One organic solvent selected from ethylene carbonate,propylene carbonate, methyl ethyl carbonate and diethyl carbonate, or amixture of two or more selected therefrom is most preferable.

The examples of the electrolyte salt used for the nonaqueous electrolytesecondary battery according to the present invention, not being limitedto a specific one, include LiClO₄, LiBF₄, LiAsF₆, CF₃SO₃Li, LiPF₆,LiN(CF₃SO₂)₂, LiN(C₂F₅SO₂)₂, LiI, LiAlCl₄ and a mixture thereof. Onelithium salt selected from LiBF₄ and LiPF₆ or a mixture of two or moreselected therefrom is preferable.

Furthermore, for the above electrolyte, a solid, ion conductive polymerelectrolyte can be used as supplement. In this case, examples of theconfiguration of the nonaqueous electrolyte secondary battery includethe combination of a positive electrode, a negative electrode and aseparator with an organic or inorganic solid electrolyte and the abovenonaqueous electrolytic solution; and the combination of an organic orinorganic solid electrolyte film as a positive electrode, a negativeelectrode and a separator with the above nonaqueous electrolyticsolution. The polymer electrolyte film composed of polyethylene oxide,polyacrylonitrile, polyethylene glycol, the modified body thereof, orthe like, is lightweight, flexible and advantageous in using for awinding electrode plate. Furthermore, in addition to polymerelectrolyte, inorganic solid electrolyte or a mixed material of organicpolymer electrolyte with inorganic solid electrolyte can be used.

Other Examples of the battery components include a current collector, aterminal, an insulating plate and a battery case. Also for these parts,those which have been conventionally employed can be used for thepresent invention without modification.

EXAMPLES

Hereinafter, a description is given for Examples 1-4 and ComparativeExamples 1-3 according to the present invention.

Example 1

[Preparation of Lithium-containing Layered Nickel Oxide Particles]

Nickel sulfate and cobalt sulfate were dissolved in a prescribedcomposition ratio; and aqueous sodium hydroxide was then added theretowhile agitating sufficiently so as to obtain nickel-cobalt combinedcoprecipitation hydroxide. The formed coprecipitate was washed withwater, dried and fully mixed with aluminum hydroxide; then lithiumhydroxide monohydrate salt was added thereto; adjustment was made sothat the molar ratio between lithium and nickel + cobalt + aluminumwould be 1.05:1 so as to prepare the precursor.

Next, the precursor was calcinated under an oxygen atmosphere at 700° C.for 20 hours; cooled down to the room temperature; and taken out indried argon gas to be crushed, so that the lithium-containing layerednickel oxide particles represented by the compositional formulaLi_(1.03)Ni_(0.85)Co_(0.12)Al_(0.03) O₂ were obtained. It should benoted that the obtained lithium-containing layered nickel oxideparticles were stored in a desiccator in air at 0.1 atm for threemonths. The average composition of the obtained lithium-containinglayered nickel oxide particles was analyzed by the inductively coupledplasma spectrometry method. In the powder X-ray diffraction, the peak ofunreacted hydroxide or impurities such as lithium aluminate was notfound.

The average particle size D₅₀ of the obtained lithium-containing layerednickel oxide particles was 9.5 μm and the particle size range was 15 μm,which were measured by the laser diffraction/scattering method. The BETspecific surface area was 0.35 m²/g, which was measured using BETmethod.

Next, in order to examine the surface properties of the obtainedlithium-containing layered nickel oxide particles, a qualitativeanalysis was carried out from the compound surface toward the inside inthe depth direction using the X-ray photoelectron spectroscopy method(XPS) and argon-ion etching in combination.

The analysis was carried out in the following procedure. First, thelithium-containing layered nickel oxide particles were applied onto aconductive carbon tape placed on a sample stage for the X-rayphotoelectron spectroscopy method in a drying room at a dew point of−50° C. or below. A stainless steel plate having a clean surface was putthereon and pressed moderately using a hydraulic presser so as toprepare a visually flat and dense sample.

Next, the above sample was put in an X-ray photoelectron spectroscopyapparatus using a transfer vessel so as not to be exposed to air. Sincethe analysis area diameter of the X-ray photoelectron spectroscopymethod was set to 100 μmΦ, the spectrum to be obtained was the averagevalue obtained from several tens of compound particles. Nevertheless,although several tens of operations were repeatedly performed using theidentical compound from the sample preparation operation up to the X-rayphotoelectron spectroscopy measurement operation, no error was includedin the obtained information.

Since gases or impurities are slightly adsorbed onto the outermostsurface of the lithium-containing layered nickel oxide particles, thespectrum after etching down to a depth of 1 nm from the outermostsurface is defined as a true surface spectrum of the lithium-containinglayered nickel oxide particle surface here. The depth was computed byconverting based on the thickness of single-crystal Si.

The successive analysis of the lithium-containing layered nickel oxideparticle surface was carried out on the flat plate on which powder wasaggregated and pressed as described above. Even when the same analysisis carried out on a polar plate formed by mixing the lithium-containinglayered nickel oxide particles with acetylene black or poly(vinylidenefluoride), and by applying the mixture onto a flat plate followed bypressing, since oxygen is not contained in acetylene black andpoly(vinylidene fluoride), there is no trouble in specifying thelithium-containing layered nickel oxide particle surface and determiningthe oxygen chemical bond state on the surface. Specifically, the sameanalysis can be carried out on the polar plate.

FIG. 4 shows the oxygen 1s spectrum of the obtained lithium-containinglayered nickel oxide particles. FIG. 4 shows that both the oxygen 1sspectrum on the surface (L=1 nm) and the oxygen 1s spectrum at a depthof L=500 nm from the surface are D>E, which represents that theconcentration of the oxygen contained in the crystal is higher than theconcentration of oxygen adsorbed anywhere in the particle regardless ofthe depth from the surface. It should be noted that there was novariation in oxygen concentration ratio at a depth of 500 nm or inner.

[Preparation of Test Battery]

A positive electrode was prepared by mixing the above lithium-containinglayered nickel oxide particles of 87 weight %, acetylene black of 5weight % and poly(vinylidene fluoride) (PVdF) of 8 weight %; addingN-methyl-2-pyrrolidone (hereinafter, referred to as “NMP”) with a watercontent of 50 ppm or below to the mixture to make a paste; applying thepaste onto an aluminum foil leaf; and drying to form a positiveelectrode plied material layer. A negative electrode was prepared bymixing carbon material (graphite) with PVdF; adding NMP to the mixtureto make a paste; applying the paste onto a copper foil leaf; and dryingto form a negative electrode plied material layer.

The belt-like positive and negative electrodes thus prepared were woundinto an ellipse through a separator so as to form an electrode group asshown in FIG. 2, and the electrode group was then inserted into the longcylindrical, closed-end aluminum case. After filling the core of theelectrode group with filling material, electrolytic solution wasinjected thereinto and the case was sealed with the lid by laserwelding. It should be noted that all the processes from pastepreparation, electrode formation up to battery assembly were carried outunder a dried condition at a dew point of 50° C. or below.

[Characteristic Test]

After charging the test battery with a current of 1CA up to 4.2V, thedischarge capacity was measured when discharging with a current of 1 CAup to 3.0 V so as to compute the initial discharge capacity of positiveactive material per g.

Next, the discharge capacity was found after charging-discharging 300cycles under the same charge-discharge conditions as the test battery soas to compute after-cycle capacity retention. It should be noted thatthe “after-cycle capacity retention (%)” here means the value obtainedby dividing the discharge capacity after 300 cycles by the initialdischarge capacity.

Furthermore, comparisons regarding preserving characteristics were madewith another battery prepared at the same time as the battery used inthe charge-discharge cycle test. Initially, after the charge-dischargeprocess was repeated three times in which charge was carried out with acurrent of 1 CA up to 4.2V followed by discharging with a current of 1CA up to 3.0V, the third discharge capacity was set to the initialdischarge capacity. Next, after charging again with a current of 1 CA upto 4.2V, the battery was stored under a condition of 60° C. for 10 days.After the storage period, the charge-discharge process was repeatedthree times under the same condition as the initial processes, and thethird discharge capacity was set to the after-storage discharge capacityso as to compute the after-storage capacity retention. It should benoted that the “after-storage capacity retention (%)” here means thevalue obtained by dividing the after-storage discharge capacity by theinitial discharge capacity.

Example 2

A precursor was prepared similarly to Example 1. The precursor wascalcinated in an oxygen atmosphere at 700° C. for 20 hours; cooled downto the room temperature; and then taken out in dried argon gas to becrushed, so that the lithium-containing layered nickel oxide particlesrepresented by the compositional formulaLi_(1.03)Ni_(0.85)Co_(0.12)Al_(0.03)O₂ were obtained. The obtainedlithium-containing layered nickel oxide particles were stored in adesiccator in a vacuum for a month. XPS measurement and batterycharacteristic measurement were then carried out similarly to Example 1.

Example 3

The lithium-containing layered nickel oxide particles represented by thecompositional formula Li_(1.03)Ni_(0.85)Co_(0.12)Al_(0.03)O₂ wereobtained similarly to Example 2 except that a precursor was calcinatedin an oxygen atmosphere at 700° C. for 5 hours. Storage, XPS measurementand battery characteristic measurement were then carried out under thesame conditions as in Example 1.

Example 4

The lithium-containing layered nickel oxide particles represented by thecompositional formula Li_(0.03)Ni_(0.85)Co_(0.12)Al_(0.03)O₂ wereobtained similarly to Example 2 except that a precursor was calcinatedin an oxygen atmosphere at 650° C. for 20 hours. Storage, XPSmeasurement and battery characteristic measurement were then carried outunder the same conditions as in Example 1.

Comparative Example 1

A precursor was calcinated and crushed under the same conditions as inExample 2 so as to obtain the lithium-containing layered nickel oxideparticles represented by the compositional formulaLi_(1.03)Ni_(0.85)Co_(0.12)Al_(0.03)O₂. After storing the precursor in adesiccator in a vacuum for 12 months, XPS measurement and batterycharacteristic measurement were carried out under the same conditions asin Example 1.

Comparative Example 2

A precursor was prepared similarly to Example 1. The precursor wascalcinated in an oxygen atmosphere at 700° C. for 20 hours; cooled downto the room temperature; taken out in dried argon gas; and then crushedin a dried atmosphere, so that the lithium-containing layered nickeloxide particles represented by the compositional formulaLi_(1.03)Ni_(0.85)Co_(0.12)Al_(0.03)O₂ were obtained. The obtainedlithium-containing layered nickel oxide particles were stored in adesiccator in a vacuum for a month. XPS measurement and batterycharacteristic measurement were then carried out similarly to Example 1.

Comparative Example 3

The same precursor as in Example 1 was calcinated and crushed under thesame conditions as in Example 2 so as to obtain the lithium-containinglayered nickel oxide particles represented by the compositional formulaLi_(1.03)Ni_(0.85)Co_(0.12)Al_(0.03)O₂. The particles were stored in airat 1 atm for one month. Storage, XPS measurement and batterycharacteristic measurement were then carried out under the sameconditions as in Example 1.

Based on the XPS measurement result of the lithium-containing layerednickel oxide particles used in Examples 1-4 and Comparative Examples1-3, Table 1 shows the value of the oxygen concentration ratio α_(L) ata depth of L (nm) from the surface; and Table 2 shows the value of(α₀−α_(L))/α₀ at a depth of L (nm) from the surface. It should be notedthat the value for α_(L) did not vary in the depth range of 500 nm orabove from the surface in any of Examples and Comparative Examples.Therefore, the value for α_(L) at a depth of L=500 nm was set to α₀.Also, the result of the battery characteristic measurement is shown inTable 3. TABLE 1 Value α_(L) at depth L (nm) from particle surface L =10 L = 50 L = 100 500 Example 1 49 68 74 80 Example 2 49 70 77 81Example 3 47 69 75 81 Example 4 48 68 74 80 Comparative 41 62 70 81Example 1 Comparative 38 55 64 80 Example 2 Comparative 35 49 60 81Example 3

TABLE 2 Value (α₀ − α_(L))/α₀ at depth L (nm) from particle surface 1050 100 500 Example 1 0.388 0.135 0.075 0 Example 2 0.395 0.136 0.049 0Example 3 0.420 0.148 0.074 0 Example 4 0.400 0.150 0.075 0 Comparative0.494 0.235 0.136 0 Example 1 Comparative 0.525 0.313 0.200 0 Exam le 2Comparative 0.568 0.395 0.259 0 Example 3

TABLE 3 Initial discharge After-cycle After-storage capacity capacityretention capacity retention Ah % % Example 1 192 80 89 Example 2 191 8190 Example 3 190 81 90 Example 4 188 82 91 Comparative 188 78 88 Example1 Comparative 188 77 86 Example 2 Comparative 187 77 87 Example 3

Based on the results shown in Tables 1-3, in the lithium-containinglayered nickel oxide particles whose composition is represented by thegeneral formula Li_(a)Ni_(x)Co_(y)Al_(z)O₂(0.3≦a≦1.05, 0.7≦x≦0.87,0.1≦y≦0.27, 0.03≦z≦0.1; b=0 in this case), it was found that batteryperformance varies depending on difference in surface propertiestypified by oxygen chemical bond state even when using a compound havingthe identical composition. Specifically, as in the batteries of Examples1-4, in the binding energy of the oxygen 1s spectrum when measuring theparticles by XPS, if the peak area appearing at 529 eV is set to D; thepeak area appearing at 531 eV is set to E; oxygen concentration ratio isset to D/(D+E); and the oxygen concentration ratio at depths of L1 nmand L2 nm from the particle surface are respectively set to α_(L1) andα_(L2), in case the combination of L1 and L2 in which(α_(L2)−α_(L1))/α_(L2)≦0.1, L1≦100, L2≧500 is present, superior batteryperformance can be obtained both in after-cycle capacity retention andafter-storage capacity retention.

On the other hand, Comparative Examples 1-3, where there is nocombination of L1 and L2 in which (α_(L2)−α_(L1))/α_(L2)≦0.1, L1≦100,L2≧500, were inferior in both after-cycle capacity retention andafter-storage capacity retention.

The following has become clear after the above examinations. Since theperformance of the battery using Ni-based compound for positive activematerial undergoes a sensitive reaction in the properties of thecompound surface, quality control is necessary on the surface in orderto obtain a good battery performance as expected, which can bedetermined based on whether or not oxygen chemical bond state is withina stipulated range.

Although the present invention has been described in detail withreference to the specific examples, it should be understood by thoseskilled in the art that various changes and modifications may be madetherein without deviating from the spirit and scope of the presentinvention.

This application is based on the Japanese Patent ApplicationNo.2003-348728 filed on Oct. 7th, 2003. The entire disclosure of thespecification is incorporated herein by reference.

INDUSTRIAL APPLICABILITY

As has been described above, since the after-cycle capacity retentionand after-storage capacity retention of a nonaqueous electrolytesecondary battery improve thanks to the present invention, the presentinvention has a significant industrial applicability.

1. A nonaqueous electrolyte secondary battery which comprises a positiveelectrode including particles of lithium-containing layered nickel oxiderepresented by a general formula Li_(a)Ni_(x)Co_(y)Al_(z)M_(b)O₂,wherein: 0.3≦a≦1.05, 0.7≦x≦0.87, 0.1≦y≦0.27, 0.03≦z≦0.1, 0≦b≦0.1; M isat least one selected from metallic elements except Ni, Co and Al, andin binding energy of oxygen 1s spectrum when measuring said particles byXPS, if a peak area appearing at 529 eV is set to D; a peak areaappearing at 531 eV is set to E; oxygen concentration ratio is set toD/(D+E); and oxygen concentration ratios at depths of L1 nm and L2 nmfrom the particle surface are respectively set to α_(L1) and α_(L2), acombination of L1 and L2 in which (α_(L2)−α_(L1))/α_(L2)≦0.1, L1≦100,L2≧500 is present.
 2. The nonaqueous electrolyte secondary batteryaccording to claim 1, wherein said particles are crushed in an argon-gasatmosphere.
 3. The nonaqueous electrolyte secondary battery according toclaim 1, wherein average particle size D₅₀ of said particles is 4-20 μm.4. The nonaqueous electrolyte secondary battery according to claim 3,wherein the average particle size D₅₀ of said particles is 9-10 μm. 5.The nonaqueous electrolyte secondary battery according to claim 1,wherein BET specific surface area of said particles is 0.1-1 m²/g. 6.The nonaqueous electrolyte secondary battery according to claim 5,wherein the BET specific surface area of said particles is 0.3-0.4 m²/g.7. The nonaqueous electrolyte secondary battery according to claim 1,wherein a combination of L1 and L2 in which−0.1≦(α_(L2)−α_(L1))/α_(L2)≦0.1, L1≦100, L2≧500 is present.
 8. Thenonaqueous electrolyte secondary battery according to claim 7, whereinthe combination of L1 and L2 in which 0≦(α_(L2)−α_(L1))/α_(L2)≦0.1,L1≦100, L2≧500 is present.
 9. The nonaqueous electrolyte secondarybattery according to claim 1, wherein 0.98≦x+y+z+b≦1.01.
 10. Thenonaqueous electrolyte secondary battery according to claim 1, wherein Mis a transition metal element except Ni and Co.